In extractive metallurgy, solvent extraction reagent formulations typically comprise oxime extractants, organic diluents, and in some cases, equilibrium modifiers and/or kinetic modifiers.
In order to operate a plant for the recovery of a metal such as copper from an ore by leaching-solvent extraction-electrowinning under optimal conditions, one generally first has to define the leaching conditions required to bring the copper metal values into an aqueous solution. Depending on the nature and grade of the ore, the resultant leach solution will contain a certain amount of, for example, copper within a certain concentration range. It will also contain a certain concentration range of residual leaching reagent, typically sulfuric acid in the case of copper. It will also contain various other metals like for example iron, magnesium, and aluminum as the sulfate. In selecting the optimum extractant and extractant concentration to use for recovery of the copper values from the leach solution, one needs to also take into consideration things such as the stripping conditions which in turn are going to be determined by how one wants to operate the tank house, the desired recovery of metal from the leach solution, the circuit configuration (number of extraction stages and stripping stages available depending on capital constraints of the project) as well as the nature of the leach solution. Proper selection of the optimum extractant is important to insure maximum profitability.
Once the reagent has been selected, it is added to the circuit as initial fill and additional fresh reagent is added as required to maintain the optimum reagent concentration over time. A certain amount of reagent is lost overtime as either entrainment of the organic phase in the exiting aqueous raffinate or by chemical degradation of the oxime necessitating the addition of fresh reagent to the organic to maintain the desired reagent concentration.
Aldoximes are strong extractants which are difficult to strip resulting in relatively poor net copper transfer in a circuit under typical operating conditions. Ketoximes are weak extractants which are very readily stripped resulting in very good net copper transfer in a circuit under typical operating conditions. The variation of the amount of ketoxime relative to aldoxime in the reagent formulation results in extractant formulations having different extractive strengths allowing the performance to be tailored to a particular application as disclosed for example in U.S. Pat. No. 4,507,268 and U.S. Pat. No. 4,544,532. Examples of non-modified ketoxime/aldoxime blends having a range of extractive strengths are LIX® 937N, LIX® 984N, and LIX® 973N
As noted above, a drawback of existing processes is the loss of organic reagent due to acid catalyzed hydrolysis, especially at higher temperatures, which leads to the formation of unwanted aldehydes or ketones and the entrainment of organic matter—oximes, modifiers and solvents—into the aqueous phase which exits the process. While ketoximes are somewhat more stable than aldoximes, the differences are not great and one finds that the relative ratios of ketoxime to aldoxime in the circuit organic remain relatively close to that of fresh reagent over time under typical plant operating temperatures and acid concentrations in the feed solution and electrolyte. Addition of fresh reagent maintains the overall balance of the extractant composition in the circuit organic very close to the optimum.
In the case of modified oximes, the situation can be quite different. As explained above, aldoximes are strong extractants that are very difficult to strip under typical operating conditions and as a result are not typically used by themselves. The relative extractive strength of an aldoxime can be varied by mixing it with different amounts of equilibrium modifiers such as described in U.S. Pat. Nos. 4,978,788; 5,176,843; 5,281,336; 6,113,804; 6,726,887, 6,177,055 and 6,231,784. Examples for formulated reagent concentrates can be found in U.S. Pat. No. 7,025,899—all these documents hereby incorporated by reference. By varying the amount of aldoxime to modifier in the organic phase, one can vary the extractive strength of the reagent formulation to match the requirements of the particular application.
One can also develop modified ketoxime/aldoxime blends for particular applications as disclosed for example in U.S. Pat. No. 7,309,474. Depending on the nature of the modifier (water solubility, volatility, chemical stability), one can encounter problems with modified aldoximes formulations. Over a period of time in operation, one can find a selective loss of one component of the reagent formulation relative to the other component from the circuit organic. Simply adding the fresh reagent as formulated may not maintain the desired reagent composition in the circuit organic. This is especially problematic in the cases where the modifier is lost at a much slower rate than the oxime due to the fact that the modifier is significantly more chemically stable, less volatile, and less water soluble than the oxime resulting in a buildup of the modifier in the circuit organic relative to the required concentration of the oxime.
TXIB, a branched di-ester (U.S. Pat. No. 4,978,788) and di-n-butyl adipate (U.S. Pat. No. 6,177,055) are currently widely used as equilibrium modifiers. They are characterized as being non-soluble in water, non-volatile and have high chemical stability in the application relative to the oximes. When using one of these di-esters as an equilibrium modifier, one finds that after a period of time, the level of modifier has greatly increased relative to the amount of aldoxime with the effect that the plant is effectively operating with a much weaker extractant than optimum and a higher modifier concentration than necessary resulting in increased viscosity of the organic phase which in turn increases physical problems such as increased aqueous entrainment, increased crud formation, and higher organic losses. It would be highly desirable to be able to adjust the ratios of the extractant components in the circuit organic in a manner that brings the extractant composition in the organic phase back to the optimum formulation initially chosen for the application while maintaining the effective reagent concentration in the desired range.